The main rotor blades' material was changed to a titanium-fiberglass composite. A new automatic flight control system was added. The vertical tail was also enlarged, with the tail rotor tilted upwards slightly to provide some lift in hover. The initial YCH-53E first flew in 1974. Following successful testing, the initial production contract was awarded in 1978, and service introduction followed in February 1981. The US Navy acquired the CH-53E in small numbers for shipboard resupply. The Marines and Navy acquired a total of 177. For the airborne mine countermeasures role, the Navy obtained a CH-53E variant, designated MH-53E Sea Dragon, with enlarged sponsons and fuel tanks for greater fuel storage, in the 1980s. The Navy obtained 46 Sea Dragons. CH-53K The USMC had planned upgrades to retain most CH-53Es, but this effort stalled. Sikorsky proposed a new model, originally designated "CH-53X"; in April 2006, the USMC signed a $18.8 billion contract for 156 "CH-53K" helicopters, with deliveries to be completed by 2021. The USMC was to begin retiring CH-53Es in 2009, and needed replacements as rotorcraft reached their structural life limits in 2011–12. CH-53K flight testing was expected to begin in 2011. In August 2007, the USMC increased the order from 156 to 227. By that time, the first flight was planned for November 2011 with initial operating capability (IOC) by 2015. The CH-53K will be the USMC's heavy lift helicopter with the MV-22 (medium lift) and UH-1Y (light lift). A 2007 RAND report on seabasing found that a higher ratio of CH-53Ks to MV-22s would reduce ship-based deployment times. In 2008, design work was well underway, along with weight reduction efforts to meet operational requirements; increased engine performance and rotor blade improvements are options to help meet requirements if needed. The rotor mast tilt was decreased and components shifted to ensure the center of gravity does not shift too far rearward as fuel is burned. Design requirements were frozen in 2009–10. On 22 January 2010, Sikorsky opened a $20 million Precision Components Technology Center in Stratford, Connecticut, for producing CH-53K parts, such as the rotating and stationary swashplates, main and tail rotor hubs, and main rotor sleeves. On 3 August 2010, the CH-53K passed its Critical Design Review, reading it for test production. However, the IOC fielding date was deferred to 2018. Sikorsky proposed building four pre-production aircraft for evaluation. On 4 December 2012, Sikorsky delivered the first CH-53K,